a) Field of the Invention
The invention is directed to a method and an arrangement for optical stress analysis of solids based on the measurement of the change in the polarization state of light passing through material.
b) Description of the Related Art
In the field of material testing for absence of stress, polarimetric measurement of stress in the test piece by means of a quantity proportional to the shearing stress has proven desirable. However, the real problem with these methods, known per se, consists in that the quantity to be measured is given by typically extremely small phase shifts (Γ<10−4) between the ordinary and extraordinary beam. In this connection, it is self-evident that such small measurement quantities are subject to a variety of disturbances that can have an unwanted influence on measurements and must therefore be suitably suppressed. Further, economically sensible evaluation of test pieces in a production process requires measurement times which are on the order of magnitude of the process cycle time, that is, as short as possible. Therefore, known technical solutions must be gauged based on the extent to which they can simultaneously satisfy the three conditions mentioned above.
The use and construction of polarimeters for detecting stress states in plates by utilizing the effect of stress induced birefringence have been described many times in the literature. A general survey of the methods currently being used is presented in J. W. Dally and W. F. Riley [Experimental Stress Analysis, McGraw-Hill, New York 1991: 424-505].
The known concepts and practical implementation of simple basic constructions of polarimeters are also shown in DE 31 29 505 A1 and DE 36 44 705 C2.
In DE 31 29 505 A1, monochromatic, circularly polarized light is passed through the test piece in a known manner and the change in this polarization state is detected as light power I by an analyzer combination comprising a λ/4 plate and a Wollaston prism, over two light output channels by means of photocells. This method gives phase shift Γ by means of the equation tan2Γ=I1/I2, where I1 and I2 are the intensities of the two light output channels. It will be seen immediately that these methods do not provide directional information about the position of the principal axes of elliptically polarized light and provide exclusively a quadratic dependence of the phase shift. Due to the quadratic dependence, the sensitivity approaches zero particularly for small phase shifts Γ and statistically equally distributed depolarizations in the material lead to erroneous measurement results in the direction of high values of the phase shift.
This lack of directional information is overcome in Patent DE 36 44 705 C2 in that linearly polarized light is used and the angle between the principal axes of index ellipsoid of the sample and the polarization direction is varied by mechanically moving elements. In a modification and expansion of the principle described in DE 36 44 705 C2, measurement methods which carry out the measurement with two linear polarization directions rotated by 45° or 90° are also known (U.S. Pat. Nos. 4,629,323; 5,521,705). However, none of these methods is capable of overcoming the disadvantages resulting from the quadratic dependence, namely, an extremely low measurement sensitivity for small phase shifts and a falsification of the measurement results in the direction of high values brought about by depolarizations which are statistically equally distributed in the material.
Another basic approach is presented by concepts which attempt to directly determine the elements of the Mueller matrix (R. M. A. Azzam, Opt. Lett. 2 (6) (1978): 148]. Known practical implementations are arranged as two-channel polarimeters in which the change in intensity of a linearly polarized light beam is detected after interaction with the sample in two receiver channels with orthogonal analysis.
A solution disclosed in U.S. Pat. No. 5,247,176 has two phase delays (retarders) which rotate synchronously at different speeds. Apart from the use of two retarders, this solution requires an extremely high optical-mechanical precision of the rotating apparatus and considerable numeric calculation because the measurement results are obtained from a Fourier analysis of the measurement signals. Although the method does not have a vanishing measurement sensitivity even with small phase shifts (around Γ=0), accuracy is limited due to the fact that higher order Fourier coefficients have great weight in the measurement results. The accurate determination of such coefficients requires a precisely synchronized and uniform rotation of the two retarders and is therefore very complicated and time-consuming.
The suitability of the solution described in DE 42 11 742 A1 depends on the exact mutual mechanical adjustment of a plurality of polarization-selective component groups which are arranged one behind the other. The quality of this adjustment directly determines the measurement sensitivity and measurement accuracy of the arrangement. This means that expenditure on adjustment rises sharply particularly for the measurement of small phase shifts (around Γ=0). Moreover, this solution is unsuitable in principle for the suppression of statistically equally distributed depolarizations in the measurement sample.
Therefore, none of the prior art solutions are capable of achieving at a reasonable expenditure a high measurement sensitivity for small phase shifts (around Γ=0) with depolarizations directed in a defined manner due to stress birefringence and, simultaneously, a suppression of depolarisation which are statistically eqally distributed in the measurement sample.